SLI and CrossFire Push Power Supplies to the Limit

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When we were writing our recent socket 939 motherboard mini-roundup, we wanted to test how the system would run with two high-end graphics cards installed. We began with a system that had a Silverstone SST-ST65ZF PSU. This power supply is rated for 650 watts, and is SLI certified by Nvidia.

So we fired up an Nforce4 SLI X16 system with two 512MB graphics 7800 GTX graphics cards and an Athlon 64 FX-60. About halfway through the 3DMark06 run, the system shut down. There was no warning, no smoke, no heat. It just turned off. After some grumbling we tried swapping out memory and changing out the CPU. No effect. So we finally switched the power supply to a PC Power & Cooling Turbo-Cool 850 SSI. At that point, it all ran just fine. We were in a rush to wrap up benchmarking the motherboards, so just chalked it up to a bad power supply and moved on.

Note that when the system would shut down, the power supply wasn’t even warm—there was no overheating issue there. Later, we replicated the problem with two ATI X1900 cards running in CrossFire mode, so this is not simply an Nvidia issue.

After the article ran, Brett Jacobs from AMD called. Jacobs works in the AMD performance testing lab, which constantly tests dozens of configurations. He had zeroed in on my comment about the system shutting down and had some interesting things to share.

According to Jacobs, AMD had started seeing similar shutdowns with high-end, dual-graphics card setups recently. He was concerned that readers might attribute the problem to the power draw of the FX-60, which would be an incorrect conclusion. Jacobs pointed the finger squarely at dual-graphics cards, noting that the 512MB 7800 GTX can draw in excess of 11 amps when running full bore. If the two cards share a power supply rail, that means that more than 20A is being pulled from one power supply rail, which is a recipe for disaster. According to another source, AMD has started recommending 700W power supplies with high-end SLI or CrossFire setups to alleviate potential problems.

The issue, by the way, has nothing to do with the CPU. AMD was seeing these problems with CPUs as low as the single core Athlon 64 3200+, which draws relatively little power. Continued…

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